1. Field of the Invention
The present invention relates to a primer composition. More particularly, the present invention relates to a primer composition which, when dental adhesive restorative materials such as dental resin cements, composite resins, polymethylmethacrylate (PMMA) resins and the like are made to adhere directly or through bonding agents to substrates such as vital hard tissues, especially an enamel or a dentin of natural teeth, glass ionomer cements and the like, is able to impart a strong and durable adhesive property between both without treating said substrates with an acid and the like.
2. Description of the Prior Art
Dental restorative resins are in general insufficient in their adhesive property to teeth such that restorative materials may fall off or that there may occur secondary caries, pulpal irritation, marginal fracture or marginal discoloration due to invasion or leakage of bacteria at the margin of restoration, causing problems in dental clinic.
To solve these clinical problems, techniques to improve the adhesive property between the restorative materials and teeth have been proposed in the prior art, as described in detail below.
A proposal of the so-called acid etching technique is now being evaluated clinically as a technique for adhesion to the enamel [Journal of Dental Research, Vol. 34(6), pp. 849-853, (1955)]. Adhesion by this technique is based not on the chemical bonding between the teeth and adhesive resins but on mechanical anchor effect caused by curing and anchoring of the resins that have penetrated into a honeycomb structure mainly formed by demineralization of enamel prisms. However, this honeycomb structure is not formed on the surface of the enamel which is not subjected to the acid etching treatment and adhesive property in this case remains still insufficient.
Recently much interest has been focused on adhesion to the dentin and many proposals have been made increasingly, with the result of some improvements. These proposals are, however, concerned with the method wherein the dentin is pretreated, or reformed and reinforced in some cases, by using inorganic acids, organic acids, organic acids and some kinds of metal chlorides, or ethylenediaminetetraacetic acid (EDTA) or their salts. Apart from the case of the enamel, clinical problems are pointed out in these methods when they are applied to the dentin. Etching treatment of the dentin especially by using phosphoric acid has caused many discussions in Japan as well as in Europe and United States, and American Dental Association expressed a recommendation against the phosphoric acid treatment of the dentin. However, citric acid and oxalic acid are yet taken into account for the acid treatment agent of the dentin since they are considered to have less etching effect than phosphoric acid [see IADR/AADR Abstracts No. 915, p. 276 (1985) or U.S. Pat. No. 4,521,550 and No. 4,538,990]. Treatment methods using EDTA are also proposed [see U.S. Pat. No. 4,553,941 and No. 4,593,054]. However, dental society or dentists desire so strongly a method yielding a strong and durable adhesive strength of the dentin without subjecting it to any acid treatment.
Meanwhile, adhesive strength to the dentin have remarkably progressed in recent years. It is reported that an adhesive strength of 100 kgf/cm.sup.2 or more was reached by using a conventional bonding material and a composite resin, wherein the dentin was treated with a mixed solution of glutaraldehyde, water and 2-hydroxyethyl methacrylate (named as Gluma) after a pretreatment with a solution of EDTA.Na salt [Journal of Dental Research, Vol. 63(8), pp. 1087-1089 (1984); Scand. J. Dent. Res., Vol. 93, pp. 463-466 (1985); and International Dental Journal, Vol. 35, pp. 160-165 (1985)]. It is proved, however, that when the dentin is not subjected to the EDTA.Na treatment, a value of about 30.9 kgf/cm.sup.2 is actually shown, indicating that the value still remains low.
In Proceedings of the 7th Meeting of Japan Society for Adhesive Dentistry, pp. 121-122 and pp. 123-124 (1989), or in Proceedings of the 1st International Congress on Dental Materials, pp. 236-237 (1989) and The Journal of Dental Engineering No. 90, pp. 31-38 (1989), it is reported that high adhesive strength is maintained by methylmethacrylate (MMA)/partially oxidized tri-nu-butylborane (TBB-O) resin even after 2000 times of thermal cycles by using cool water (4.degree. C.) and hot water (55.degree. C.), wherein the dentin has been subjected to, a primer treatment using a mixed solution of solution A [70% of 2-hydroxyethylmethacrylate (2-HEMA)+6% of o-methacryloxytyrosinamide (MTYA)] and solution B (2% of glutaraldehyde), which is so-called above-described Gluma type of treatment solution, after an acid treatment of the dentin. For example, it is reported that an adhesive strength of 164 kgf/cm.sup.2 is obtained by a mixed primer treatment of the above-described solution A and solution B after a treatment with 40% of phosphoric acid. On the other hand, an adhesive strength of 48 kgf/cm.sup.2 to the dentin without acid treatment is shown even at 0th cycle, which is indicative that not only the power is insufficient in a sense of the intrinsic adhesive strength but also durable adhesive property to the dentin without acid treatment is hardly obtained. Although adhesive strength is improved by this method, it is still low against the dentin without acid treatment in a strict sense. The proposer of the above-described technique has recognized in the disclosures that the intrinsic adhesive strength to the dentin has not been reached yet.
More recently, a primer composition which does not require any pre-treatment such as acid treatment, i.e. a primer composition for hard tissues comprising water, a water soluble film-forming agent and salts of acids, has been proposed (see Japanese Patent KOKAI No. 113057/1989).
When the primer composition is used for making a composite resin adhere to the dentin of teeth, however, the adhesive strength is largely influenced by the kinds and blending amounts of acid salts, thereby causing difficulties that not only the resulting adhesive strength may be inferior to that obtained by using no primer composition, but also durable adhesive property is not obtained.